PROJECT ABSTRACT We propose to examine the impact of recreational marijuana legalization on the development of marijuana use and marijuana use disorders, alcohol, nicotine, and other substance use and use disorders, and associated psychological adjustment and psychopathology. To do this, we will compare outcomes in two population based twin samples that have been followed longitudinally for over 15 years since adolescence. The age ranges of the samples at will be 23-39 at their follow-up assessment, thus, we will be able to examine the impact of legalization on early- to mid-adulthood functioning of family formation, work engagement, and adult role fulfillment. We will obtain additional waves of data in these samples in order to examine the effect of legalization on rates of marijuana, alcohol and other substance use and use disorders as well as associated psychological adjustment and psychopathology. One of these samples is from Colorado, which legalized adult recreational marijuana use in 2014 and now has widespread commercial marijuana readily available to consumers. The other sample is from Minnesota, which in 2014 legalized medical marijuana, with very strict limits on access, and has no recreational legalization. Together with our detailed prior longitudinal data, the new assessments, post-legalization of substance use, psychopathology, and psychosocial functioning in over 5000 population-based adult twins will provide unique and powerful data to understand the impact of recreational marijuana legalization and marijuana use on a wide variety of important outcomes. Furthermore, the effect of recreational marijuana legalization is unlikely to be uniform across the population. Rather, some individuals will be at higher risk to suffer negative consequences of RML, such as increased use and dependence. Leveraging the longitudinal and twin structure of the study, we will be able to determine the influence of pre-legalization individual differences in behavioral risk and psychosocial function on the effect of recreational marijuana legalization.